The present invention relates to surface modification of fabricated or semifabricated parts and more particularly to producing changed physical or chemical properties on metal, e.g. hardened surfaces.
There are many known and long practiced methods for improving the resistance of fabricated or semifabricated metal (including elements, alloys and compounds) to wear, galling, deformation, corrosion, heating and/or erosion. These include overcoating the surface of the metal and modification of the composition and/or microstructure of the surface through such techniques as carburizing, nitriding, siliconizing, diffusion hardening, hard surfacing (welding a high-alloy layer to the surface), flame hardening, induction hardening and physical modification (e.g. peening). The overcoating methods include electroplating chromium or nickel on to the surface, plasma spraying or flame spraying refractories on to the surface and roll cladding (for sheet form or wire form mill products).
The state of the art with respect to the commercial methods in most widespread commercial use for such purposes is given in the report, "Hard Facing by Arc Welding", by the American Society for Metals, Committee on Hard Facing, appearing in the Eighth edition of the ASM Metals Handbook at pages 152-166. This describes the application of oxyacetylene welding, shielded metal-arc welding, open-arc welding,, gas tungsten-arc welding, submerged-arc welding and plasma arc welding, to hard facing, including description of the form of hard facing alloy (rod, powder, wire), weld metal dilution ratios, (varying from 1 to 10 for oxyacetylene, 10 to 30 for most of the other methods, but as high as 60 for submerged-arc welding), and other parameters. Related information is given in U.S. Pat. No. 3,678,374 to Hauck, assigned to Union Carbide Corporation and in the article by Union Carbide Corporation by Zuchowski and Gerrabrant, "New Developments in Plasma Arc Weld Surfacing", Welding Journal, vol. 41, No. 6, pp. 548-555 (1962) and in U.S. Pat. No. 3,819,901 to Berinde, et al. It is a common problem of such methods that the costly hard facing alloy must be pre-prepared and used in excessive quantities because of the need to establish minimum thicknesses for effective deposition and to grind down the hard facing coating to final dimension. The substrate part which is hard faced is often undesirably affected through incidental heat treatment as a result of the hard facing process. All such methods involve dilution of hard facing material and establish composition of resultant surface in less controlled manner than is desirable. All such methods are also relatively intolerant of geometry since they involve placing arc torches or electrodes adjacent the region to be filled and in that the precision of the close spacing is critical.
It has been proposed to melt the surface layer of a metal substrate by electron beam impingement to react with a coating material to form a hard surface of exceptional adhesion properties in the proceedings of the Apr., 1966 Conference on "Electron and Ion Beams Science and Technology" of the American Institute of Mining, Metallurgical and Petroleum Engineers (Vol. 51, Metallurgical Society Conference, page 605). Electron beams tend to drill through and provide deep penetration characteristics when applied at power density levels sufficient for rapidly melting a surface layer but may be defocussed to limit deep penetration. However, such beams tend to charge a coating of metal or other conductive particles negatively and, through repulsion between such particles themselves the coating particles would be scattered from the surface before a substantial portion of the reaction could occur. Electron beam technology is also limited by the need for ambient vacuum or very close spacing between a work piece and an exit orifice of an out-of-vacuum electron beam system since ambient air will scatter an electron beam.
It has also been proposed to accomplish hard facing by coating a substrate surface and applying the beam of a pulsed ruby laser to melt the surface layer in the article by Schmidt, "Tools and Engineering Materials with Hard Wear-Resistant Infusions", Journal of Engineering for Industry 549-552 (August, 1969). Surface alloying using a pulsed semiconductor laser is proposed in articles by Mirkin and Gazuko at Doklady Akad. Nauk SSSR, May 11, 1969, Volume 186, No. 2, pages 305-308 (an English translation appearing in Soviet Physics -- Doklady, Volume 14, No. 5, pages 494-496 of 1969) and in Poroshkovaya Met. of January 1974, Part 1, pages 27-30. The Schmidt proposal results in a composite of tungsten carbide in a matrix of substrate sub-surface material (typically iron). None of the Schmidt, Mirkin and Gazuko articles describe commercially implemented processes. The limitations of these proposals may be understood by the context of the art given in the exhaustive review article by Gagliano, et al, "Lasers in Industry", Proceedings of the Institute of Electrical and Electronic Engineers, Volume 57, No. 2, 1969, pages 114-147.
Pulsed semiconductor lasers applied along a linear work path produce a series of shallow craters and a rough undesirable product surface. After grinding to smooth such a surface, the alloy layer may be removed in part.
It is an important object of the present invention to provide an improvement in metal wear resistance protection and related arts in respect of extending the method capabilities of such art(s) and/or producing improved products.
It is a further object of the invention to convert a surface layer on a substrate to a form in which it is highly adherent to the substrate material consistent with the preceding object.
It is a further object of the invention to convert a surface layer on a substrate to a wear resistant form which includes a significant proportion of the substrate as a separate and distinct phase and/or as a constituent of a newly formed compound, consistent with the preceding objects.
It is a further object of the invention to convert a surface layer on a substrate to a wear resistant form made by melting and mixing the surface layer with alloying and/or reactive materials in a very short time, consistent with the preceding objects.
It is a further object of the invention to deal with irregularly shaped materials consistent with one or more of the preceding objects.
It is a further object of the invention to provide chemical modification of surfaces consistent with one or more of the preceding objects.
It is a further object of the invention to provide high density, low porosity surface layers consistent with one or more of the preceding objects.
It is a further object of the invention to utilize standard equipments borrowed from other major purposes and not necessarily dedicated to surface modification consistent with one or more of the preceding objects.
It is a further object of the invention to provide surface layer treatment which is tolerant of difficult geometries, including re-entrants and remote surface regions, consistent with one or more of the preceding objects.
It is a further object of the invention to provide surface layering, without regard to electrical or magnetic field conditions which may exist in the region or surface to be treated or which may develop in the course of processing, consistent with one or more of the preceding objects.
It is a further object of the invention to utilize low cost materials, with respect to initial selection and quantity and in limitation of quantity of usage, consistent with one or more of the preceding objects.
It is a further object of the invention to minimize the costs in labor, materials and/or time of ancillary machining and/or heating steps related to surface layering, consistent with one or more of the preceding objects.
It is a further object of the invention to provide controlled dilution of alloying material and/or establish alloy composition with a high degree of control, consistent with one or more of the preceding objects.
It is a further object of the invention to provide flexibility of process control consistent with one or more of the preceding objects.
It is a further object of the invention to minimize incidental effects on the substrate below the surface layer consistent with one or more of the preceding objects.
It is a further object of the invention to provide areally homogeneous surface layers at any given depth in respect of composition and/or microstructure consistent with one or more of the preceding objects.
It is a further object of the invention to provide minimal working time and related substrate preparation and posttreatment times consistent with one or more of the preceding objects.